lavr: call mix_function_init() in ff_audio_mix_set_matrix()

This is needed if a custom matrix is set by the user after opening the
AVAudioResampleContext because the matrix channel count can change if
different mixing coefficients are used.

CC:libav-stable@libav.org

libavresample/audio_mix.c

@@ -422,10 +422,6 @@ AudioMix *ff_audio_mix_alloc(AVAudioResampleContext *avr)
         av_free(matrix_dbl);
     }
 
-    ret = mix_function_init(am);
-    if (ret < 0)
-        goto error;
-
     return am;
 
 error:
@@ -704,5 +700,5 @@ int ff_audio_mix_set_matrix(AudioMix *am, const double *matrix, int stride)
         return AVERROR(EINVAL);
     }
 
-    return 0;
+    return mix_function_init(am);
 }